1. Field of the Invention
The present invention relates to a hologram and more particularly to a volume phase hologram.
Hologram, that is, a recording member for recording the interference fringe of light is expected to be widely used in information processing, image indication and optical measurement.
2. Description of the Prior Art
Hologram is classified into two types, that is, a phase type of hologram and an amplitude type of hologram. It is theoretically confirmed that the maximum diffraction efficiency of amplitude hologram is only 7.2%.
The phase hologram may be generally assorted further into two types according to the value of Q of the following equation: EQU Q=(2.pi..lambda.d/n.sub.o .DELTA..sup.2)
The equation was defined by H. Kogelnik wherein .lambda. is wavelength of light, d is thickness of photosensitive layer n.sub.o is average refractive index and .DELTA. is synchronism of interference fringe. For the phase hologram with its Q being at least 10, it is called volume type. When Q is smaller than 10, the phase hologram is called planer type. A higher diffraction efficiency can be obtained from the volume type one. The maximum diffraction efficiency of the volume type phase hologram is theoretically 100%. When photopolymer is used for producing such a volume type phase hologram, the thickness of its sensitive layer becomes necessarily large up to a value from 10 .mu.m to hundreds .mu.m. If dichromate gelatine is used, a volume type phase hologram may be produced by a sensitive layer with the thickness less than 10 .mu.m (several .mu.m).
As hologram recording material, there are known photochromic material, chalcogen glass, thermoplastics, silver salts, photoresist, photopolymer, dichromate gelatine, ferroelectric material and ferromagnetic material. All these material have both merits and demerits. Therefore, those materials are very few which are able to satisfy the requirements for practical use.
With the exception of silver salts, dichromate gelatine and photopolymer, the above mentioned materials have a particular disadvantage that they lack sensitivity to visible rays and therefore their use is limited only to ultraviolet range. Although dichromate gelatine has visible sensitivity, it is not allowed to use any other than chromate, which in turn sets limits to the range of wavelength to which the material is sensitive. Silver salt has a sufficient sensitivity for being used as a hologram material. However, there is a problem of reduction in resolving power and transmission factor (transmittivity) caused by the graininess of silver. In particular, darkening due to its poor light fastness after bleaching is a serious defect of this material. A dichromate gelatine hologram is disclosed in U.S. Pat. No. 3,617,274 and a bleached silver salt hologram in U.S. Pat. No. 3,672,744.
Also, photopolymer has some disadvantages. In order to attain a high diffraction efficiency, it is required to make the sensitive layer extremely thick as described above. Furthermore, in order to obtain the necessary visible sensitivity, there are used sensitizers. These sensitizers remain unchanged in the product and thereby some problems such as decrease of transmission factor and coloring are caused.